1. Field of the Invention
This invention relates to phase-type focusing screens and, more particularly, to phase-type focusing screens which, when used in, for example, the photographic camera or the like to form a finder image thereon, have a predetermined diffusing characteristic so that the finder image can be observed in good quality.
2. Description of the Related Art
There has been a previous proposal for forming a great number of minute patterns of a constant height and a predetermined shape on the surface of an optically transparent substrate so that the light beam passing through the minute patterns is given a phase difference relative to the other light beam to provide the focusing screen with a predetermined diffusing characteristic, i.e., a so-called bi-level form of the phase-type focusing screen, in, for example, Japanese Laid-Open Patent Application No. Sho 55-70827 and Japanese Laid-Open Patent Application No. Sho 62-269127.
FIG. 9 is a schematic perspective view of the main part of the phase-type focusing screen of the bi-level form shown in the specification of Japanese Laid-Open Patent Application No. 55-70827. In the same figure, reference numeral 101 denotes a substrate plate. Minute patterns 102 of a constant height are formed on the surface of the substrate plate 101. And, a light beam which passes through a region A constituting part of the surface of the substrate plate 101 and another light beam which passes through a region B constituting all the patterns 102 are given a phase difference to obtain a predetermined diffusing characteristic.
The focusing screens proposed in these publications, because of using the periodic array of the minute patterns, suffer from a strong influence of diffraction depending on the wavelength of light, so that the image the photographic system forms, when blurred, has color deviation. Another problem is that because the intensity distribution of the diffracted rays has discretely displaced peaks, those portions of the formed image which are out of the depth of focus give an unpleasant impression.
Particularly with the bi-level phase-type focusing screen, the degree of color stain in the image of the 0th, .+-.1st, .+-.2nd and higher order diffracted rays is roughly determined by the distance between the two levels or the depth (phase) of the concave and convex profiles. To correct this was a hard task. For the 0th order light intensity, the possible minimum is limited to 4-5%. To decrease the minimum from this value was, in the general case, another very difficult problem.
As related art, there is U.S. patent application Ser. No. 359,487 filed on May 31, 1989.